Of the xylene isomers, para-xylene (often abbreviated PX) is of particular value since it is useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers. Equilibrium mixtures of xylene isomers either alone or in further admixture with ethylbenzene generally contain only about 24 wt % para-xylene and separation of p-xylene from such mixtures has typically required superfractionation and multistage refrigeration steps. Such processes have involved high operation costs and resulted in only limited yields.
Crystallization technology has been used to purify paraxylene commercially. The paraxylene streams currently purified using crystallization are not made via alkylation of toluene with methanol and thus do not contain significant concentrations of oxygenates.
During the past decade or more an improved process of toluene alkylation has been developed using as catalysts certain porous crystalline materials, preferably having specific and closely defined diffusion characteristics, such as can be obtained by unusually severe steaming of ZSM-5 containing an oxide modifier. Under appropriate conditions, these catalysts exhibit improved selectivity for the alkylation of toluene with methanol such that the xylene product contains at least about 90% of the paraisomer isomer at per-pass toluene conversions of at least about 15%. This important development has been described in numerous patents and publications, such as U.S. Pat. Nos. 4,002,698; 4,356,338; 4,423,266; 5,675,047; 5,804,690; 5,939,597; 6,028,238; 6,046,372; 6,048,816; 6,156,949; 6,423,879; 6,504,072; 6,506,954; 6,538,167; and 6,642,426.
The paraxylene-rich stream produced by alkylation of toluene with methanol using the aforementioned steamed porous crystalline material has both high concentrations of C8 aromatics (o-xylene, m-xylene, styrene, ethylbenzene, etc.) and low but significant concentrations of oxygenated species (acetic acid, phenol, cresols, etc). Previous processes of making paraxylenes by alkylation did not have significant quantities of oxygenates, accordingly this problem and its magnitude was unexpected.
There are a number of possibilities that come to mind in purifying these feedstreams once the impurities have been identified.
For instance, one could use a combination of processes to first remove low concentrations of oxygenates (e.g. extraction, absorption, etc.) and then a second process to remove high concentrations of C8 aromatics (e.g. simulated counter-current absorption). However, this is energy and time-inefficient.
A more efficient process is required to purify the paraxylene to meet requirements for use in subsequent chemical processes (e.g. production of terephthalic acid).
The present inventors have surprisingly discovered a process whereby, using crystallization technology, paraxylene may be purified by separating paraxylene both from concentrations of other C8 aromatics and also small concentrations of oxygenated species.
Crystallization technology per se is well-known, and there are numerous permutations of this technology. See, for instance, U.S. Pat. No. 7,439,412. There are also numerous licensed commercial crystallizer technologies, such as the Amoco Process, Maruzen Process, Esso Process, Sulzer Chemtech Process, and the like.
Fractional crystallization in a crystallizer takes advantage of the differences between the freezing points and solubilities of the C8 aromatic components at different temperatures. Due to its higher freezing point, PX is usually separated as a solid in such a process while the other components are recovered in a PX-depleted filtrate. High PX purity, a key property needed for satisfactory commercial conversion of PX to terephthalic anhydrode (PTA) and/or dimethyl terephthalate (DMT) in most plants, can be obtained by this type of fractional crystallization. U.S. Pat. No. 4,120,911 provides a description of this method. A crystallizer that may operate in this manner is described in U.S. Pat. No. 3,662,013.
The present inventors have surprisingly discovered a process using crystallization technology to purify paraxylene having a small but significant quantity of oxygenated species.